The present invention relates to establishing the size of servo zones in a disk drive. In particular, the present invention relates to establishing the size of servo zones in a disk drive having a partial response maximum likelihood type detector.
Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks that are divided into servo sectors and data sectors. Information is written to and read from a disk by a transducer head, mounted on an actuator arm capable of moving the transducer head radially over the disk. Accordingly, the movement of the actuator arm allows the transducer head to access different tracks. The disk is rotated by a spindle motor at a high speed, allowing the head to access different sectors on the disk. The transducer head may include separate or integrated read and write heads.
The servo sectors provide information concerning the position of the transducer head with respect to the surface of the disk. In particular, the servo sectors may provide information concerning the particular track and the sector on that track that is being addressed by the transducer head. Because servo sectors are interspersed between data sectors, the disk drive receives information concerning the position of the transducer head over the disk surface intermittently. Typically, the number of servo sectors per track at an inner diameter is the same as the number of servo sectors per track at an outer diameter.
In order to reliably read position information from the servo sectors on a disk, the position information is commonly encoded as xe2x80x9cGray code.xe2x80x9d In Gray code, only one bit changes between successive code words. By using Gray code, errors can be minimized if it is known that only one bit in the code word is changing.
The signal derived from servo sector Gray code will vary according to its position on the disk. In particular, the amplitude of a servo sector signal derived from a servo sector located at an outer diameter of the disk surface will be greater than the amplitude of a signal derived from a servo sector at an inner diameter of the disk. This variance is due to intersymbol interference. In order to properly decode signals derived from servo sectors associated with tracks at different locations across the disk, adjustments may be made to the detection circuit or algorithm. In particular, different filters or equalizations may be provided to account for expected changes to the signal produced in the transducer head by different servo sectors according to the radius of the track with which the servo sector is associated. For example, the tracks on a disk surface may be considered as a series of zones, each containing the same number of tracks. The detection circuit may adjust, for example, the amount by which a signal is amplified according to the zone in which the servo sector being read is located. In particular, the gain in the detection circuit may be increased for tracks located in a zone at an inner diameter of a disk as compared to a zone located at an outer diameter of a disk. The use of zones allows the detector to accommodate differences in the signal produced by servo sectors at different locations on the disk, while avoiding the complexity of providing a different gain amount or equalization scheme for each track on the disk.
One type of detector that may be used to decode the position information stored in servo sectors is a partial response maximum likelihood (PRML) type detector. In general, PRML type detectors increase the accuracy of the detection circuit, because they allow a sequence of signals or pulses to be considered together, rather than as a series of discrete pulses. The sequence of pulses is then matched to a most likely bit sequence to arrive at a code word. Accordingly, PRML type detectors consider the shape of an entire sequence of pulses. PRML type detectors also allow information to be written at higher bit densities, because the intersymbol interference that results from closely spaced pulses can be accommodated better than with peak detection methods, which require detection of a discrete pulse at each sampling interval.
With respect to servo sector information, which is written at a constant frequency across all the tracks of the disk, the radius of the track from which servo sector information is read may influence not only the amplitude but the shape of the pulse produced in a transducer head by a servo sector. Using conventional zoning methods, such as dividing the tracks into a number of zones having an equal number of tracks, the detection of Gray code using PRML techniques is less than optimal. For instance, where the tracks on the disk are considered as a plurality of zones having an equal number of tracks, the detection of servo sector information may be unreliable in certain of the tracks within a zone. Accordingly, it may be necessary to further subdivide zones, and provide a unique signal conditioner for each of the subzones. Using conventional zoning methods, subzones are created by dividing the number of tracks in a zone by the desired number of subzones to obtain subzones that each have an equal number of tracks. Therefore, the code required in order to implement a PRML system for detecting servo sector information according to conventional methods is relatively complex. This complexity may result in decreases in the performance of the drive, and may require more sophisticated processors and more internal memory in the detector circuit than would otherwise be required. In addition, the large number of zones required to achieve satisfactory detector performance results in increased manufacturing times, because there are a large number of zones to test on each drive.
Accordingly, a need exists for a method of dividing the tracks of a disk drive into servo sector zones that allows for the detection of servo sector information with improved accuracy and reduced overhead requirements. In addition, it would be advantageous to provide a disk drive apparatus having a PRML type detector for detecting servo sector information with improved drive yield. Furthermore, it would be advantageous to provide a method and apparatus that could be implemented at an acceptable cost, and that would be reliable in operation.
In accordance with the present invention, a method is provided for sizing servo sector zones to optimize the equalization of signals derived from servo sector information in a computer disk drive. In addition, the present invention provides an apparatus having servo sector zones sized so as to optimize the equalization of signals derived from servo sectors. According to the present invention, servo sector zones are sized such that each zone encompasses a like range of channel bit densities.
In accordance with one embodiment of the present invention, a range of channel bit densities across the surface of a magnetic disk in a computer disk drive is calculated. The disk is then divided into a selected number of servo sector zones, each encompassing a like range of channel bit densities. A single signal equalization scheme may then be provided for the detection of signals derived from a single corresponding zone. By sizing servo sector zones to encompass like ranges of channel bit densities, an equal or near equal range of servo sector signal variations is encountered in each zone. This in turn enables servo sector information to be correctly decoded using only a single equalization scheme per zone. Furthermore, the present invention reduces the mean square error of the detector within the borders of any one zone, resulting in increased manufacturing yield.
According to one embodiment of the present invention, the equalization applied to signals derived from a particular zone is optimized for the middle channel bit density value of the zone. According to a further embodiment of the present invention, the equalization applied to signals derived from a particular zone is optimized for a track located in the middle of the zone. According to still another embodiment of the present invention, the equalization of signals derived from a zone is optimized for a track having a channel bit density equal to the average channel bit density of the tracks included in the zone.
Based on the foregoing summary, a number of salient features of the present invention are readily discerned. A method for optimizing the equalization of signals derived from servo sectors on a disk drive is provided. Furthermore, a disk drive apparatus having optimized equalization of signals derived from servo sector information and increased manufacturing yield is provided. The method and apparatus of the present invention allow signals derived from servo sector information to be optimally conditioned for a given number of servo sector zones and a corresponding number of equalization parameters.